Germany is in the EU and trades very successful with China, India, Brasil, Russia, US, ...
http://atlanticsentinel.com/2011/03/bric1-germanys-new-bedfe...
http://www.reuters.com/article/2012/04/23/us-germany-china-i...
If the UK can't do that, it is not due to the EU, but because of local incompetence. Germany has high growth rates in trade with the countries and a huge export surplus (even with China).
If you narrow yourself to the trade and banking within the EU, yes I agree, Germany are bigger player than anybody else. Start thinking globally, and you'll soon realize that UK stands much better chances in global economy trading freely with the Commonwealth and Asia than turning their back to these markets and focusing on the local European block.
Think Hong Kong, think Canada, think Australia -- from UK perspective it is just better business to be in an free trade economic block with them then to stay in euro disaster zone. This is what they have been doing for centuries. UK is no.1 in the world in terms of value of items traded per capita. Yes, they are a nation of shop-keepers. EU seems to be one-size-fits-all solution that obviously doesn't work for them anymore and probably never worked. They'll exit, open borders to Asia, ex-colonies, trade with them, do their banking, logistics and all the other stuff they have been doing centuries. While Germany will still export/import in Hamburg, nobody minds, everybody just does what's the best for their country. No surprises there.